1. Field of the Invention
This invention relates generally to optical coherence tomography and tunable lasers.
2. Description of Prior Art
Optical coherence tomography (OCT) is a technique for high-resolution depth profiling of a sample below the sample surface. OCT systems include a broadband light source, means for directing radiation from that light source to a reference mirror and to a sample, and means for detecting interference of light reflected from the reference mirror with light reflected from the sample. In time-domain OCT (TDOCT), the broadband light source is typically a superluminescent diode, which simultaneously emits multiple wavelengths, and the position of the reference mirror is scanned to depth profile the tissue. In swept source OCT (SSOCT), a tunable laser is employed as the broadband source. Here only one wavelength is present at any one time, and sweeping of the laser wavelength replaces mechanical scanning of the reference mirror. In recent years, SSOCT has become more widely employed, because of its theoretically demonstrated signal to noise ratio advantage over TDOCT, as described in (M. A. Choma, M. V. Sarunic, C. Yang, and J. A. Izatt, “Sensitivity Advantage of Swept Source and Fourier Domain Optical Coherence Tomography,” Optics Express, vol. 11, no. 18, Sep. 8, 2003, p. 2183-2189.).
FIG. 1 illustrates a prior art SSOCT system. FIG. 1 employs a tunable source 100, which comprises a gain/cavity region 110, a bottom distributed Bragg reflector (DBR) mirror 120 and a top Fiber Fabry-Perot (FFP) mirror 130. An optical pump 140 pumps the tunable source 100, either through the mirrors or from the side. A tunable output radiation 150 of the tunable source 100 is split to a reference mirror 160 and to a sample 170, using a fiber coupler 180. Interference between light reflected from the reference mirror 160 and the sample 170 is detected by an optical detector 190. The optical power detected by 190 as a function of the wavelength of the radiation 150 is related by a Fourier transform to the depth profile of the sample reflectivity. The prior art configuration of FIG. 1 is described in detail in (J. Zhang, J. S. Nelson, and Z. Chen “Removal of a mirror image and enhancement of the signal to noise ratio in Fourier Domain Optical Coherence Tomography using an electro-optic phase modulator,” Optics Letters vol. 30, no. 2, Jan. 15, 2005).
Other researchers have employed different means to achieve swept sources for SSOCT, including edge-emitting external cavity lasers, or a ring laser source tuned by a fiber Fabry-Perot filter, as in (Michael A. Choma, K. Hsu, and J. Izatt, “Swept Source Optical Coherence Tomography using an all-fiber 1300 nm ring laser source,” Journal of Biomedical Optics 10(4) July/August 2005). All of the tunable lasers used in prior art SSOCT systems suffer from one or more of the following problems: slow wavelength scanning limited by mechanical constraints or long cavity lifetime, incomplete wavelength coverage, or limited tuning range. The scan rate for tuning schemes that employ FFP filters as one mirror of a cavity, for example, is limited to a few kilohertz (kHz). The limitation on scan rate ultimately leads to long image acquisition times, making the SS-OCT system less attractive for clinical use. Limited tuning range translates into limited spatial resolution, and incomplete wavelength coverage translates into more shallow depth information.
Aside from the tunable lasers described above, other candidate tunable lasers for SSOCT include widely tunable lasers designed for telecommunications. One example of such a laser is the sampled grating DBR (SGDBR) laser described in (B. Mason, S. Lee, M. E. Heimbuch, and L. A. Coldren, “Directly Modulated Sampled Grating DBR Lasers for Long-Haul WDM Communication Systems,” IEEE Photonics Technology Letters, vol. 9, no. 3, March 1997, pp. 377-379). The SGDBR is a multi-section laser requiring control of 4 electrodes to set the wavelengths. The wavelength is a complex and non-monotonic function of these wavelengths, and this laser is not suitable for the kind of linear or sinusoidal wavelength sweeping required in SSOCT systems.
From the foregoing, it is clear that what is required is an SS-OCT system employing a tunable laser with wide tuning range, wavelength scan frequency greater than a few kHz, complete wavelength coverage over the tuning range, and wavelength tuning that is a simple monotonic function of a tuning control signal.